Copper backed electrical contact

ABSTRACT

A copper backing is formed on a silver-cadmium oxide contact by first forming a bimetal strip having a thin layer of substantially pure silver and copper layer. The bimetal strip is secured at at least one end to a billet of the contact material, to retain them together during subsequent operations, with the silver surface of the bimetal strip against a surface of the billet of contact materials. The composite is then rolled to a substantial reduction of thickness, of the order of at least 50 percent to bond the silver surface of the bimetal to the contact material. This produces a tri-layer contact with the layers fully integrated, consisting of the main contact material section, an intermediate silver layer and a copper backing layer.

United States Patent 1191 Backstrom Nov. 27, 1973 COPPER BACKED ELECTRICAL CONTACT [75] Inventor: Melvin L. Backstrom, Murrysville,

21 Appl. No.: 205,273

[52] US. Cl. 29/195, 29/199 [51] Int. Cl B32b 15/00 [58] Field'of Search 29/195 C, 195 M,

[56] References Cited UNITED STATES PATENTS 1,986,224 Sanders 3,668,758 6/1972 Krock 29/199 Primary Examiner--Hyland Bizot Attorney-Sidney G. Faber et al.

[5 7] ABSTRACT A copper backing is formed on a silver-cadmium oxide contact by first forming a bimetal strip having a thin layer of substantially pure silver and copper layer, The bimetal strip is secured at at least one end to a billet of the contact material, to retain them together during subsequent operations, with the silver surface of the bimetal strip against a surface of the billet of contact materials. The composite is then rolled to a substantial reduction of thickness, of the order of at least 50 percent to bond the silver surface of the bimetal to the contact material. This produces a tri-layer contact with the layers fully integrated, consisting of the main contact material section, an intermediate silver layer and acopper backing layer.

5 Claims, No Drawings Because of the affinity of copper for oxygen, heating in an oxidizing atmosphere results in the formation of 5 copper oxide which is an insulator. On the other hand,

the kinetics of the silver-cadmium oxide are such that if heated in a neutral or a reducing atmosphere, decomposition occurs. The cadmium oxide decomposes at about 1,400F. anddecomposition can occur at temperatures even as low as 500F. depending on the time of exposure in a reducing atmosphere.

Toovercome this apparent incompatibility, the present invention provides a bimetal strip of copper with a fine silver backing. The silver surface of the bimetal is placed in contact with the silver-cadmium oxide and the bondable surfaces are co-rolled to provide a composite electrical contact having a silver-cadmium oxide layer, a fine silver layer not more than and preferably l%5%, of the total thickness of the contact, and the copper backing layer. The copper backing of the tri-level lay ercontact can be readily secured to a contact support. l H

The primary object of this invention is therefore the provisionv of a tri-level contact element having a layer of contact material such as silver-cadmium oxide, a pure silver intermediate layer, and a copper backing, the silver layer being firmly bonded to the copper backing and also being firmly bonded to the silver-cadmium oxide layer thereby providing a contact structure which can be readily secured to a contact support in a device intended to control an electrical circuit.

Another object is the provision of a method of making the tri-layer contact which includes the steps of providing a bimetal silver-copper member, and securing the silver surface of said bimetal member to a contact material such as silver-cadmium oxide.

Still another object of the invention is the provision of a brazing or soldering surface for a silver-cadmium oxide or other silver based composite which alone is not readily attached by brazing or soldering.

The foregoing and other objects of this invention will become apparent in the following detailed description.

In accordance with the present invention, there is provided an electrical contact material and a bimetal backing strip. The contact material can be silvercadmium oxide in which the cadmium oxide is generally up to about 50 weight percent. However, the brittleness of the contact material increases with the percentage of the cadmium oxide and it is therefore preferred to employ a material in which about 25-15 weight percent is cadmium oxide and the balance is silver.

The bimetal backing strip has a layer of silver and a layer of a material such as cbpper which is readily soldered or brazed. The silver surface of the bimetal backing strip now provides the means for attaching the bimetal strip to the contact and the copper surface provides the readily brazable or solderable surface which can be used to attach the contact to contact support structures or electrical devices.

The composite silver-copper strip can be formed by any suitable manner as by cold or warm bonding by rolling, by pressure, by plating or other means which will form the bimetallic strip. A method which is currentiy employed in the metal industry is hot-roll bonding. ln'this method, the metals to be joined are preheated and subjected to high localized forces by passing thestructure through a rolling mill. As a result of the combination of elevated temperature and rolling pressure, a metallurgical bond is obtained. Any desired thickness of copper and silver can be employed provided' that the silver thickness allows a satisfactory bond between the silver and silver-cadmium oxide when the trilevel contact element is mechanically worked.

The surface of the contact material which is to be se- I cured to the bimetal strip and the silver surface of the bimetal strip are thoroughly cleaned in a manner well known in the art. The bimetal strip and contact material are contacted in a manner to provide continued alignment during processing. This can be accomplished by mechanically forming the bimetal so as to provide a physically locked total configuration, by spot-weld tacking, or otherwise mechanically fixing the bimetal backing to the silver-cadmium oxide before initiating the bonding. I

The bonding of the bimetal backing strip and the silver-cadmium oxide contact material is effected by mechanically working the opposing faces together. Because silver-cadmium oxide cannot be heated in a reducing or neutral atmosphere and copper cannot be heated in an oxidizing atmosphere, it is preferable to perform the initial bonding operation by rolling the composite structure to obtain a reduction in thickness at ambient temperature. The structure is rolled to a reduction of not less than 50% taking as many passes as necessary, although it is possible to obtain this reduction in one pass. The purpose of the rolling operation is to enable a complete metallurgical bond and it has been found that a good metallurgical bond can only be achieved if an initial reduction of at least 30% is effected by the first pass through the rolls.

If desired, a preliminary heating operation can be performed to increase the ductility of the contact mate rial and insure that during the rolling operation which takes place immediately thereafter, the materials will not separate and no thin spots will occur. A temperature of 500750 F. for a period of time selected so as to avoid substantial decomposition of the heated materials is usually employed. If the contact material is silver-cadmium oxide, the heating operation can take place in air. If the contact material is of other structure (such as silver tungsten) the heating operation can take place in a reducing atmosphere such as hydrogen.

A typical copper backed electrical contact of the present invention is prepared as follows. Slabs of a composition having silver and 10% cadmium oxide were cut into inch x 1% inches X 6 inches sections. A commercially available bimetal strip of coldbonded fine oxygen-free, high conductivity silvercopper having a total thickness of 0.060 inch and a fine silver layer of 0.006 inch was cut to size and mechanically bent to form a V- shape. The contact material and bimetal strip were mechanically cleaned by sanding and mechanically affixed together. The total thickness was 0.560 inch. The composite was fed with the aid of roll guides into l4 inches diameter work rolls set to provide an opening between rolls of 0.250 inch. The rolling speed was feet per minute and the contact angle was approximately 4. After the first pass through the rolls, a strip approximately 1% inches wide by l 1% inches long with a thickness of 0.256 inch was obtained.

The resulting strip was evaluated for bond strength and brazing characteristics. A section of the strip was placed in a vice and bent into a 90 bend which resulted in the breaking of the silver-cadmium oxide section but the integrity of the bond was sufficient to resist peeling of the backing member from the main structure. Subsequent chiseling at the interface resulted in breaking the silver-cadmium oxide but not in destroying the bond. The brazing test was conducted by cutting several contact sections A inch X A inch 0.256 inch and brazing the sections to a backing member by induction heating. A commercial brazing alloy known as Sil-Fos was employed as the brazing media. No separation of the interfaces resulted from the heat applied during brazing. Metallographic examinations were made and verified the integrity of the multiple bonded cross-section.

The tri-level contact was then rolled to form better cross-sections. The initially bonded strips were placed in an air atmosphere furnace and heated to about 750 F. for minutes. The rolls of the rolling mill were preheated to about 100 F. Some oxidation of the copper occurred but the bonded areas had sufficient integrity to prevent deterioration of the bond surfaces. The rolls were set to provide a reduction for the pass at the same rolling speed as the initial pass. The rolled material had a thickness of 0.202 inch. Subsequent passes were made through the rolling mill using the same heating cycle but the individual reductions were reduced to about l0% per pass to eliminate the possibility of exceeding the maximum tensile strength of the silvercadmium oxide. The final thickness of the tri-level composite was 0.050 inch of which 0.006 inch was the copper backing and 0.001 inch was the fine silver intermediate layer.

Various changes and modifications can be made in the process and product of this invention without departing from the spirit and the scope thereof. The various embodiments disclosed herein serve to further illustrate the invention but were not intended to limit it.

I claim:

1. A tri-layer electrical contact consisting essentially of a layer of silver-cadmium oxide, an intermediate layer of substantially pure silver of not more than 10% of the total thickness of the contact, and a layer of copper, the substantially pure silver layer and the copper layer having been a bimetallic strip and the silver surface of the bimetallic strip being metallurgically bonded to the silver-cadmium oxide layer by rolling of the composite of the silver-cadmium oxide material and the bimetal material.

2. The tri-layer electrical'contact of claim 1 wherein the intermediate layer of substantially pure silver is l to 5% of the total thickness of the contact.

3. The tri-layer electrical contact of claim 1 wherein the layer of silver-cadmium oxide contains up to about 50 weight percent cadmium oxide.

4. The tri-layer electrical contact of claim 3 wherein the layer of silver-cadmium oxide contains 2.5-15 weight percent cadmium oxide.

5. The tri-layer electrical contact of claim 1 wherein the layer of silver-cadmium oxide contains 2.5-15 weight percent cadmium oxide and wherein the substantially pure silver layer is 1-5% of the total thickness of the contact.

k IF IF 

2. The tri-layer electrical contact of claim 1 wherein the intermediate layer of substantially pure silver is 1 to 5% of the total thickness of the contact.
 3. The tri-layer electrical contact of claim 1 wherein the layer of silver-cadmium oxide contains up to about 50 weight percent cadmium oxide.
 4. The tri-layer electrical contact of claim 3 wherein the layer of silver-cadmium oxide contains 2.5-15 weight percent cadmium oxide.
 5. The tri-layer electrical contact of claim 1 wherein the layer of silver-cadmium oxide contains 2.5-15 weight percent cadmium oxide and wherein the substantially pure silver layer is 1-5% of the total thickness of the contact. 